This invention relates to optical elements and to apparatus including such elements.
The invention is more particularly, but not exclusively concerned with lenses or windows for radiation pyrometers or other optical apparatus liable for fouling.
Radiation pyrometers used on gas-turbine engines for sensing, for example, the turbine-blade temperature, conventionally have an optical head that is mounted to project through the engine by-pass duct to the turbine chamber. The head is directed to receive radiation from the turbine blades, and in this respect includes a lens or other optical window within a sighting tube that opens through the wall of the chamber. The use of the sighting tube ensures tha radiation is received throughout a limited angle only, and also that the window is not exposed directly to the heat and combustion products within the chamber.
One problem with such pyrometers is that the lens or other window is liable to become fouled by combustion or other products during running of the engine, thereby reducing transmission of radiation.
Air can be supplied to the sighting tube to purge it of combustion products and this does to some extent reduce fouling. Problems of fouling, however, do still arise, especially during start up or shut down of the engine when the pressure of purging air may be reduced, or when the purging system becomes blocked or otherwise damaged. In some engines it may not be possible to provide a purging system and this can make it necessary to remove the pyrometer to clean the lens after only some thirty to one hundred hours of operation.